Conventionally, as a focus detection apparatus for a camera, a focus detection apparatus based on a so-called phase difference detection scheme is known (Japanese Patent Laid-Open No. 09-054242), which detects the defocus amount of an object to be photographed by forming light beams from the object, which have passed through different exit pupil areas of a photographing lens, into images on a pair of line sensors and obtaining the amount of displacement between the relative positions of a pair of object images obtained by photoelectrically converting the object images (which operation will be referred to as phase difference calculation hereinafter), and drives the photographing lens on the basis of the defocus amount.
As a focus detection apparatus of this type, a multifocus detection apparatus is known (Japanese Patent Laid-Open No. 2003-215442), which performs focus detection with respect to a plurality of objects to be photographed by segmenting a pair of line sensors into a plurality of areas, performing signal accumulation control for each area, and performing correlation calculation for a pair of object images obtained by photoelectric conversion in the respective areas.
In addition, a focus detection apparatus based on the phase difference detection scheme is known (Japanese Patent Laid-Open No. 63-172206), in which the defocus amount that can be detected can be adjusted by changing areas of a pair of line sensors which are used for accumulation control and phase difference calculation.
The focus detection apparatus disclosed in Japanese Patent Laid-Open No. 63-172206 can select proper accumulation control areas in accordance with a focus detection result and the maximum defocus amount of a photographing lens. If, however, focus detection cannot be performed, the accumulation control areas must be changed, and accumulation operation and calculation operation must be done again. This prolongs the time required for focus detection.
The focus detection apparatus disclosed in Japanese Patent Laid-Open No. 63-172206 can be applied to a multifocus detection apparatus, such as the focus detection apparatus disclosed in Japanese Patent Laid-Open No. 2003-215442, which can detect the focuses of a plurality of objects to be photographed. In this case, if accumulation control areas are small, no adjacent areas overlap. If, however, accumulation control areas are large, adjacent areas overlap. Since accumulation control cannot be simultaneously performed for overlapping areas, re-accumulation operation is performed for each area, resulting in a longer time required for focus detection.